As performance required for a filter unit, there is a low pressure drop showing whether large amount of air flow is possible to be processed or not, with a capturing efficiency. An air rate penetrating through a filtering medium becomes higher year and year; it has previously been around 15 m/min, and recently, a pressure drop as usual is required in filtration at a high air rate of 25 m/min to 40 m/min which is 1.5-fold or more. For example, in a small filter unit for air conditioning, performance of 40 Pa (capturing efficiency according to dust-spot efficiency is 90%) or less is required, and in a cabin filter which is a filter unit for automobile air conditioning, performance of approximately 85 Pa (capturing efficiency of 15 kinds of dusts by JIS are around 91%) or less is required.
In addition, as to a filter unit, thinning is required, and a thin filtering medium having a low crest height by miniature-pleating has been developed as a filtering medium for processing large amount of air flow. However, the previous filtering medium has a large thickness, and has a problem that ventilation resistance caused by a material itself of the filtering medium is high and elongation easily occurs by an air pressure. For this reason, due to ventilation resistance caused by a material itself of the filtering medium and ventilation resistance caused by deformation of the filtering medium caused by an air pressure and by choking of a flow path interval through which the air flows (hereinafter, referred to as a structure pressure drop), in a filter for automobile air conditioning at a filter size of 210 W×270 L×10 D and a processing air flow of 520 m3/hr for example, a low pressure drop filter unit having performance of a capturing efficiency of 8 kinds of JIS dusts described in JIS 28901 (1974) obtained on the basis of JIS B9908 (2001)-format 3 test method, such a dust retaining amount that the filter can be used for one year, and a pressure drop of 70 Pa or less cannot be obtained.
As the previous technique of obtaining a low pressure drop filter, there is Patent Document 1 concerning a filtering medium in which an electret sheet having specified performance is laminated on a support medium having bending resistance of 250 mg or more at 65° C. This document describes an idea that a support in which fibers are fixed with a resin having a high glass transition temperature of 35° C. is used so that a filtering medium of a filter unit attached to an automobile left under the scorching sun in the summer is not softened or deformed by heat to increase a structure pressure drop, and an idea that a capturing efficiency is increased by using lamination with an electrification sheet excellent in a quality factor. However, the filtering medium described in this document absolutely is the one preventing a structure pressure drop due to thermal deformation from being increased, but not a filtering medium which prevents a structure pressure drop due to a high air rate from being increased.
Here, in Example 3, a filter unit assembled by subjecting a filtering medium in which an electretted melt blow non-woven fabric is laminated on a net-like support having a lattice interval of 4 mm to pleats processing at a crest height of 38 mm is described, and the effect that a pressure drop of 56 Pa is attained at a filtering air rate of 37 m/min is also described. This could be unitized since the crest height is sufficiently large as compared with the lattice interval of a net; however, there is a problem that when production of a thin-type filter unit having a crest height of around 8 mm, and a crest pitch interval of 3.5 mm, disturbance in the crest pitch interval is easily caused by interference with a lattice interval of a net and, in that case, a structure pressure drop is increased, and an air flow is concentrated only on where it has large pitches. In addition, in an aspect described in this Example, since a support is a net, dust retention is low, and a dust is directly attached to an electretted melt blow non-woven fabric having a more compact structure than that of the net, increase in a pressure drop due to choking is large; therefore, shortening of a life cannot be avoided.
On the other hand, a trial to make a support medium thin and hard to suppress occurrence of a structure pressure drop is shown in Patent Document 2, and application of a paper making method using together with a modified cross-sectional yarn of a rayon fiber and a glass fiber as a fiber constituting a support is disclosed. However, a filtering medium in which a thick glass fiber is mixed has a problem of maintenance and hygiene since a glass fiber is cracked in a pleats processing step and is flown. Further, as seen from Examples, due to the use condition in which a filtering medium passing air rate is extremely slow as 2 m/min, even at a bending resistance of around 158 mg, the filtering medium could be used as a filtering medium causing little occurrence of a structure pressure drop, but for example, under a high air pressure of a filtering medium penetrating air rate of 25 m/min or more, the filtering medium could not stand sufficient use.
In addition, in Patent Document 3, there is the description regarding a HEPA filtering medium which is made by mixing extremely thin glass fibers having a fiber diameter of 0.65 micron. However, a filtering medium using a fiber having such a small fiber diameter causes a high pressure drop due to the material, and cannot be adapted to high air rate filtration at all.
Further, Patent Document 4 and Patent Document 5 disclose that dimensional stability and strength against an air pressure are improved by constructing a filtering medium with a fiber having a high Young's modulus.
Among them, Patent Document 4 relates to a filtering medium for a bag filter using a heat resistant fiber such as polyphenylene sulfide having a Young's modulus of 20 cN/dtex or more. However, this filtering medium has a shape of a woven fabric or a web, and had a remarkably different filtering medium construction in fiber usage, a structure, a weight, a thickness and using utility from a filtering medium having no loose property in which fibers are fixed.
In addition, Patent Document 5 discloses a process for producing a flame-retardant filtering medium in which the medium is produced through a paper making method using a polyvinyl alcohol fiber and a polyester fiber; it describes that, as the characteristic of a paper making method, fibers having different thicknesses and lengths can be used and that a fiber having a high Young's modulus can be also used. However, this document only describes that when the paper making method is used, a fiber having a high Young's modulus can be used, but does not disclose at all a filtering medium having such a stiffness that the medium is less likely to be deformed even under the high air rate filtering while having high air permeability such as air permeability of 100 cm3/cm2·sec or more (further, such as 250 cm3/cm2·sec or more) as well as its fiber construction and fiber physical properties.
Thus, a practical filtering medium containing mainly an organic fiber, which has high air permeability, can stand high air rate filtering, and has little elongation, has been not present.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2004-82109
Patent Document 2: JP-A No. 2002-1020
Patent Document 3: JP-A No. 10-180020
Patent Document 4: International Publication WO 0487293
Patent Document 5: JP-A No. 2006-136809
Patent Document 6: JP-A No. 2-61195